no, vacant means empty. on the other hand, futile means worthless/fruitless

No. Vacant refers to a space being empty. It is often use in hotels or motels where they will put out a “no vacancy” sign. Futile refers to action or a state of being. For example, resistance is futile means resistance is ineffective.

You can also say 'a vacant stare/ a vacant glance' which is seeing without thinking, and thus a bit futile or distracted. English is versatile and there may be some occasions when futile and vacant are synonyms, but not common.